Low noise and low power are essential characteristics for hearing aid microphones. Most high performance microphones, and particularly miniature microphones, consist of a thin diaphragm along with a spaced apart, parallel back plate electrode; they use capacitive sensing to detect diaphragm motion. This permits detecting the change in capacitance between the pressure-sensitive diaphragm and the back plate electrode. In order to detect this change in capacitance, a bias voltage must first be imposed between the back plate and the diaphragm.
This voltage creates practical constraints on the mechanical design of the diaphragm that compromise its effectiveness in detecting sound. Specifically, inherent in the capacitive sensing configuration are a few limitations. First, viscous damping caused by air between the diaphragm and the back plate can have a significant negative effect on the response. Second, the signal to noise ratio is reduced by the electronic noise associated with capacitive sensing and the thermal noise associated with a passive damping. Moreover, due to the viscosity of air, a significant source of microphone self noise is introduced. Third, while the electrical sensitivity is proportional to the bias voltage, when the voltage exceeds a critical value, the attractive force causes the diaphragm to collapse against the back plate.
To illustrate the limitations imposed on the noise performance of the read-out circuitry used in a capacitive sensing scheme, consider the buffer amplifier having a white noise spectrum given by N volts/√Hz. If the effective sensitivity of the capacitive microphone is S volts/Pascal then the input-referred noise is N/S Pascals/√Hz.
In a conventional capacitive microphone, the sensitivity may be approximated by:
                    S        =                                            V              b                        ⁢            A                    hk                                    (        1        )            where Vb is the bias voltage, A is the area, h is the air gap between the diaphragm and the back plate, and k is the mechanical stiffness of the diaphragm.
For purposes of this discussion, assume that the resonant frequency of the diaphragm is beyond the highest frequency of interest. The input referred noise of the buffer amplifier then becomes:
                              N          S                =                              Nhk                                          V                b                            ⁢              A                                ⁢                      pascals            /            MHz                                              (        2        )            
Theoretically, this noise can be reduced by increasing the bias voltage, Vb, or by reducing the diaphragm stiffness, k. Unfortunately, these parameters cannot be adjusted independently because the forces that are created by the biasing electric field can cause the diaphragm to collapse against the back plate. In a constant voltage (as opposed to constant charge) biasing scheme, the collapse voltage is given by:
                              V          collapse                =                                            8              27                        ⁢                                          kh                3                                            ɛ                ⁢                                                                  ⁢                                  A                  0                                                                                        (        3        )            where ε is the permittivity of the air in the gap. Diaphragms that have low equivalent mechanical stiffness, k, have low collapse voltages. To avoid collapse, Vb<<Vcollapse.
Equation 3 clearly shows that the collapse voltage can be increased by increasing the gap spacing, h. Increasing h, however, reduces the microphone capacitance, which is inversely proportional to the nominal gap spacing, h. Since miniature microphones, and particularly silicon microphones, have very small diaphragm areas, A, the capacitance tends to be rather small, on the order of 1 pF. The small capacitance of the microphone challenges the designer of the buffer amplifier because of parasitic capacitances and the effective noise gain of the overall circuit.
For these reasons, the gap, h, used in silicon microphones tends to be small, on the order of 5 μm. The use of a gap that is as small as 5 μm introduces yet another limitation on the performance that is imposed by capacitive sensing. As the diaphragm moves in response to fluctuating acoustic pressures, the air in the narrow gap between the diaphragm and the back plate is squeezed and forced to flow in the plane of the diaphragm. Because h is much smaller than the thickness of the viscous boundary layer (typically on the order of hundreds of μm), this flow produces viscous forces that damp the diaphragm motion. It is well known that this squeeze film damping is a primary source of thermal noise in silicon microphones.
The optical sensing approach hereinafter described is intended to be used with the microphone diaphragms described in Cui, W. et al., “Optical Sensing in a Directional MEMS Microphone Inspired by the Ears of the Parasitoid Fly, Ormia Ochracea”, January, 2006. These diaphragms incorporate carefully designed hinges that control their overall compliance and sensitivity. By combining the inventive optical sensing approach with these microphone diaphragm concepts, miniature microphones can be manufactured with extremely high sensitivity and low noise. Low noise, directional miniature microphones can be fabricated with high sensitivity for hearing aid applications. Incorporation of optical sensing provides high electrical sensitivity, which, combined with the high mechanical sensitivity of the microphone membrane, results in a low minimum detectable pressure level.
Although optical interferometry has long been used for low noise mechanical measurements, the high voltage and power levels needed for lasers and the lack of integration have prohibited the application of this technique to micromachined microphones. These limitations have recently been overcome by methods and devices as described by Degertekin et al. in U.S. Pat. No. 6,567,572 for “Optical Displacement Sensor,” copending U.S. patent application Ser. No. 10/704,932, filed by Degertekin et al. on Nov. 10, 2003 for “Highly-Sensitive Displacement Measuring Optical Device”, and copending U.S. patent application Ser. No. 11/297,097, for “Displacement Sensor”, filed by Degertekin et al. Dec. 8, 2005, all hereby incorporated by reference in their entirety.
It is, therefore, an object of the invention to provide a MEMS differential microphone having enhanced sensitivity.
It is another object of the invention to provide a MEMS differential microphone having optical means for converting sound-induced motion of the diaphragm into an electronic signal.
It is an additional object of the invention to provide a MEMS differential microphone exhibiting a first order differential response to provide a directional microphone.
It is a further object of the invention to provide a MEMS differential microphone having a silicon membrane diaphragm and protective front screen fabricated using silicon micro-fabrication techniques.
It is yet another object of the invention to provide a MEMS differential microphone having low power consumption.
It is a still further object of the invention to provide a MEMS differential microphone suitable for use in hearing aids.
It is another object of the invention to provide a MEMS differential microphone using a optical interferometer to convert sound impinging upon the microphone to an electrical output signal.
It is an additional object of the invention to provide a MEMS differential microphone wherein the optical interferometer is implemented using a miniature laser such as a vertical cavity surface emitting laser (VCSEL).